The present invention relates generally to web advancement, and it is directed particularly to sheet-feed tractors of the type used on printers and similar apparatus.
Previous advances in the tractor art have focused on the simplification of the tractor itself and on the improvement of the performance of its traditional functions. In contrast, the present invention advances the art by enhancing printer capability while at the same time permitting printer simplification.
Conventional web-feed tractors are typically employed in pairs as part of an attachment for a printer to adapt it to advancement of continuous-feed paper having edge perforations. The printer itself usually includes a drive mechanism for advancing single-sheet paper frictionally. In some printers, the friction-feed mechanism includes a pair of rollers that are spaced below a flat platen that backs the paper where the print head strikes it. In other printers, it includes a cylindrical platen, which is rotated by a drive mechanism in the printer. An idler roller grips the paper between it and the platen, and the rotation of the platen thereby advances the paper.
Particularly when a pair of rollers separate from the platen is used, the printer cannot print all the way to the bottom of the page. Even printers in which the platen itself is used for paper advancement leave a gap at the bottom of the page in which they cannot print. And both types of printers are unable to advance the sheet out of the print area when printing on the sheet has been completed.
This problem is solved by the use of the tractor attachment. The tractor attachment provides mechanical engagement of the drive mechanism whereby the tractors, which engage the edge perforations of the web, are driven by the printer drive mechanism, and they thereby advance the web. The tractors are adjustably mounted on a support shaft spaced above the platen so that they can accommodate webs of different widths. Since the tractors are spaced above the platen and pull the web from above, they can continue to advance the paper to the bottom edge of a single sheet and beyond. The tractors can thereby advance a completed page out of the print area so that the next page can be printed. In the past, however, tractors have been limited to use with continuous edge-perforated paper.
An object of the present invention is to enable the printer to print all the way to the bottom of the page when it is advancing single-sheet paper and to remove the single sheet from the print area when printing on it has been completed. A further object is to eliminate the need for friction-feed rollers in the printer itself by means of a simple addition to a conventional tractor.